MO has become specialized in combinatorics, graph theory, and algebra; if you didn't go deep in these areas you may feel at a loss. But additionally, the level of formality and carefulness in discourse is considerably higher than SO or HN. It's more reminiscent of LTU.
What? No, there's also plenty of algebraic geometry, category theory, algebraic topology, etc. Maybe not as often, but you can also find other topics, like differential geometry, set theory and logic, etc. What you feel it specializes in is probably a result of selelctive attention (depending on which questions you look at most), unless of course, you have some statistics to back up your claim (in which case, I'd still say there's enough activity in other fields for it not to matter).
